Per Ray's description, R29 is supposed to lead to some hysteresis to the comparator, I would expect this to extend the pulse width, but the scope shot is mighty narrow at the time scale you're using.

You might want to verify that R29 is indeed 1M (apologies if you already have and mentioned it previously, I was hiking around botanical gardens all day ). You might also check that it has good continuity (no cold solders, no shorts) to the two pins of U3 (5 & 7) and the two sides of C2 that it connects to.

What does the clock pin 14 on the 4017 look like when you are able to successfully trigger it with the step switch? What peak voltage, what pulse width?

A distraction. I looked into poor vertical trace on oscilloscope, in short I think my scope has a fault which is why the pulse(s) in the scope shots are so faint. I believe its still just usable. I ran it in test mode and got hardly any vertical trace. The height of the vertical trace is important not necessarily its strength?

I will confirm pulse widths and peak voltages results in next post. I did check the cathode of D18 with the scope and got +5v but I want to double check it.

Recap. On cold solder I have swapped out: R29,C2,R23,R28,R32.D17,D18. This leaves D16, I have also swapped out the socket of U3. Plus I checked all replacement parts with multimeter before soldering.

I'm not giving up hope javascript:emoticon('') but seem to be running out of options in U3b area. Am I right in thinking the issue is why I am only getting -12v/+5v on pin 7 U3 and therefore missing or have lost +7v on this output?

A distraction. I looked into poor vertical trace on oscilloscope, in short I think my scope has a fault which is why the pulse(s) in the scope shots are so faint. I believe its still just usable. I ran it in test mode and got hardly any vertical trace. The height of the vertical trace is important not necessarily its strength?

Yes, height gives the peak voltage, and width gives us a time base to know if the pulse is long enough to work. I don't know the exact requirements of the 4017, but they're in the data sheet.

Cato wrote:

I will confirm pulse widths and peak voltages results in next post. I did check the cathode of D18 with the scope and got +5v but I want to double check it.

Recap. On cold solder I have swapped out: R29,C2,R23,R28,R32.D17,D18. This leaves D16, I have also swapped out the socket of U3. Plus I checked all replacement parts with multimeter before soldering.

I'm not giving up hope javascript:emoticon('') but seem to be running out of options in U3b area. Am I right in thinking the issue is why I am only getting -12v/+5v on pin 7 U3 and therefore missing or have lost +7v on this output?

Yah, whatever is causing it to not be able to go above 5V is my current operating theory. That's enough to reset the FET (so we have oscillation) but perhaps not enough to clock the 4017 with a 12V supply.

min pulse width, btw, for a 10V supply is 90ns, and the higher the voltage the shorter the pulse can be. For a 15V supply, it's 60ns, so I'd guess 12 is probably around 80ns.

It looks like the chip wants a "high" signal to be at least .7 of the supply voltage, so we'd need to exceed 8V to get the pulse to go. That's partly why I'm curious what the pulse looks like when you successfully use the step button--what is the peak you get then versus when it's not clocking from the oscillator.

A thought that occurs to me is, if the U3 circuit resets too quickly, the pulse may not only be narrow, but also may not have time to reach +12V. From Ray's notes: "The reset pulse from the comparator is quite narrow due to the fact that Q11 turns on so quickly and brings U3-A's output below U3-B's threshold. But that squirt of charge into C5 keeps Q11 on a bit longer".

C5 is shown as 1nF. I wonder if that is in fact marginal. I'd suggest swapping it for a 10n cap (actually, just put a 10n in parallel, no need to desolder). It's possible that might cause problems at high clock speeds, but if it gets the pulse clocking the 4017 we'd be getting somewhere! (I had a similar problem recently with a reset pulse to a 4017, where the circuir called for a 10pF cap, but I found I needed 100pF to get it to work, for this reason.)

Thank you both for input need to get back to you asap.
I am still trying to find the time to investigate your recent feedback. I scoped D18k as it goes straight to pin 14 of the 4017. In start mode this gave +5v, with the "forced" sequencing via pin 5 U3 I get +7v. Will look into swapping out C5 1nf for a 10nf.

Thanks for input Chartreuse-J, I have looked at your post several times over the past couple of months.
I tried Peters idea with C5 :

C5 is shown as 1nF. I wonder if that is in fact marginal. I'd suggest swapping it for a 10n cap (actually, just put a 10n in parallel, no need to desolder). It's possible that might cause problems at high clock speeds, but if it gets the pulse clocking the 4017 we'd be getting somewhere! (I had a similar problem recently with a reset pulse to a 4017, where the circuir called for a 10pF cap, but I found I needed 100pF to get it to work, for this reason.)

No change.

I am wondering if its a spec variation in IC brands, that has tolerence(s) on the edge of the limits, which is why it works for some and not others? I found this curious post

I am going look at specs for TL072, check what I have used against other brand specs. Can't see why my issue would be the 40106 at this stage, though comparing a 'TI' to a 'Fairchild' I see a lot of differences in the specs.

Looked at u3b again,soldered a 22 swg wire to pin 5 , licked my fingers and pressed on exposed end of wire - got automatic sequencing. So I am supplying enough charge to get it working.
What is pin 5 connected to? R29,R23 and C2. Started to think do these component values need tweaking? Tried R29 first increasing and decreasing capacitance, a bit of erratic sequencing but no sound result.
Then looked at C2 again (see page 2 of thread) thought I'd touch a higher value cap 1000pf (code 103) against the existing 10pf and IT WORKED!

Swapped out 10pf for 1000pf and as far as I can visually tell everything works. I'm now going to scope test the outputs and breadboard a basic VCO to see if I can voice the notes.

Considering building the Thomas Henry VCO as I had quick success with his VCA design using veroboard.

Thank you for the guidance,advice and interest given by all of you. Especially to Elmegil - you have taught me a lot.

It's still got issues.('') - I know CV outs are working as I got scope peaks and meter voltages. Built a simple 555 VCO, to voice the pulses.
Have had 30 minutes of fun actually listening to it but only with the CV outputs, portamento works, step selector switch works as do all start/stop, step and reset buttons, CV out range selector works and so does the clock.

Typo error in previous post of mine. A 103 cap is 10 add on the 3 zeros and you get 10000pf not 1000pf as I stated earlier.
This is now irrelevant as I got to thinking 10000pf is a big value change. Took out C2 again and tried a 22pf no luck, I then tried a 22pf in parallel with a 10pf. Success it auto sequenced, so 32pf is fine. Which brings it a lot closer to Rays' spec of 10pf. I could only guess at the reason for this, slight tolerance variation in ICs? C2 at 10pf near its limit? I wonder if any one else out there had same issue?

The chips I have used are ;

ST electronics TL074CN GK735299

TI CD4013BE 13AMS42 E4

TI CD4017BE 1AD59LK E4

TI TL072CP 18MDSHY

TI CD40106BE 12AMS30 E4

Have you got a scope? Scope testing the outputs of the ICs will tell you if they are working, don't know your level of expertise. I'm definitely no expert! I would try leaving the Ics in and try C2 first.

You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot vote in polls in this forumYou cannot attach files in this forumYou can download files in this forum

Please support our site. If you click through and buy from our affiliate partners, we earn a small commission.